This invention relates to capacitors with minimum ESR, and more particularly to correlated electrolytic capacitors in a range of capacitances and voltages with minimum ESR, and a method therefor.
The dielectric of an aluminum electrolytic capacitor is aluminum oxide that is formed to a predetermined thickness by anodizing a high purity aluminum foil. This oxide on the anode foil is separated from the cathode in the capacitor section by paper spacers that are saturated with a conducting liquid electrolyte. Anode and cathode terminations are brought out from the capacitor section by means of aluminum tabs that are fastened to the foils and to the capacitor terminals.
The equivalent circuit of an electrolytic capacitor has a conductance which is the DC leakage current path, in parallel with a pure capacitance. This combination is in series with an equivalent series resistance (ESR) and an inductance. Capacitance is determined mainly by the surface area of the anode foil and the dielectric thickness. Anode foil is etched to achieve surface area gains as much as 40 times that of an unetched foil. Dielectric oxide thickness is a function of the capacitors's rated voltage. Inductance is determined by the number and position of input and output current paths that are provided to the capacitor. Inductance is influenced by various methods of connecting tabs to terminals, extended foils, and terminal positioning. The ESR of the capacitor is the sum of the resistance due to foil resistivity, oxide, electrolyte and spacer contributions, and method of termination.
It has been the practice in capacitor manufacture to reduce the ESR by increasing the foil length. However, this practice has the shortcoming that there is a limit to the extent to which increase of foil length reduces ESR. Lengthening the foil increases the foil resistance, so that at a critical point this foil resistance can result in an increase in ESR in the capacitor.
In certain capacitors, such as those used in high frequency applications, low series resistance is an important characteristic. Further, it is valuable to provide a capacitor having the minimum ESR accurately designated for use in applications of the capacitor. The application is simplified if the deviation of ESR and capacitance of the capacitor lie within a given and symmetrical tolerance.
Capacitors are used to provide capacitance in electrical circuits through a range of capacitances and at various D-C working voltages. It is established practice to provide groups of capacitors at various working voltages, so that a circuit designer has a selection of voltages and capacitances to choose from. As pointed out above, it is important for the designer to also know the minimum, maximum, and typical ESR of the selected capacitors.
Selection of a capacitor for use in an apparatus cannot disregard the dimensional requirements of the apparatus, so that the correlated units, while providing a range of capacitances, should not then encounter assembly problems.
It is an object of this invention to provide a number of capacitors of a D-C working voltage with a range of capacitances and a standard capacitor body diameter and the minimum ESR for each capacitance in the voltage rating group.